Polyisocyanate, which is used as a raw material of polyurethane, is industrially produced, for example, by allowing polyamine and carbonyl chloride to react under a solvent.
In a polyisocyanate production plant, first, in a solvent recovery chamber, the solvent is removed from a reaction solution obtained by the above-mentioned reaction to obtain an unpurified polyisocyanate, and subsequently, in a distillation column, the unpurified polyisocyanate is purified by removing tar ingredients from the unpurified polyisocyanate (see, for example, the following Patent Document 1).
Further, in the distillation column, low boiling impurities and high boiling impurities are removed from the unpurified polyisocyanate from which tar ingredients have been removed (hereinafter referred to as crude polyisocyanate) to obtain a product polyisocyanate.    Patent Document 1: Japanese Unexamined Patent Publication No. 2006-281083